<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="https://unpkg.com/vue"></script>
</head>
<script>
    window.onload= function () {
Vue.component('my-component', {
template: '<div>A custom component!</div>'
})

new Vue({
el: '#example'
})
        </script>
<body>
<div id="example">
    <div>A custom component!</div>
</div>
</body>
</html>